In the past, as a polarizable electrode of an electric double-layer capacitor, usually activated carbon or fibrous activated carbon has been used, but this has the problem that the electrodischarge capacity is small and therefore it is not possible to maintain a discharge over a long period of time at the time of actual use.
To solve this problem, Japanese Patent Publication (A) No. 7-201676 and Japanese Patent Publication (A) No. 2002-25868 propose to prepare a conductive polymer/carbon composite by electrolytic polymerization of a conductive polymer in a suspension of activated carbon (or carbon) and use the resultant composite as the polarizable electrode of an electric double-layer capacitor. They use a polyaniline/carbon composite as a polarizable electrode in working the inventions. According to these, there is the advantage that, compared with the case of use of a conventional polarizable electrode, the specific electrostatic capacity is larger and the internal resistance becomes smaller. However, electrolytic polymerization has the problem that the area of the electrode thus obtained is limited, so polymerization over a large area is difficult and not suitable for industry. Further, Japanese Patent Publication (A) No. 2002-25865 proposes to chemically polymerize aniline in the presence of a carbonaceous material in an aqueous solution so as to obtain a polyaniline/carbon composite and to use this as a polarizable electrode, but the polyaniline/carbon composite thus obtained has to be rinsed in water, so there is the problem that the operation becomes troublesome.
In addition, according to Japanese Patent Publication (A) No. 2002-25865, a composite is formed such that a carbonaceous material is impregnated with aniline and then the aniline is polymerized to form polyaniline on the carbonaceous material and, therefore, the small pores of the carbonaceous material is reduced and the remarkable improvement in the electrostatic capacity becomes difficult.
Further, according to Japanese Patent Publication (A) No. 2003-17370, it is proposed to mix a polyaniline sulfonic acid, an electrode active substance (e.g., activated carbon) and a carbon-based conductive substance in water, then distill off the mixing solvent, i.e., water, in vacuo to obtain a polyaniline/carbon composite and use the resultant composite, as a capacitor electrode. However, in the capacitor using a water-based electrolyte solution, the water-soluble polyaniline sulfonic acids is easy to leachout from the electrode and, therefore, there is the problem of inferior long term stability, as an electrode for a capacitor. Further, in the capacitor using an organic solvent-based electrolyte solution, due to the high affinity of polyaniline sulfonic acids with water, it is not possible to completely remove the water used at the time of production of the electrode from the electrode and, therefore, there are the problems of a decrease in drive voltage and inferior long term cycle characteristics. Further, there is also the problem that the sulfonic acid groups of the side chains of the polyaniline sulfonic acids cause the decrease in the drive voltage. Further, undoped polyaniline (i.e., emeraldine base-type) can be dispersed and dissolved in N-methyl-2-pyrrolidone (NMP), and, therefore, it has been proposed to mix a polyaniline/NMP solution and electrode active substance (e.g., activated carbon) and carbon-based conductive substance, then to heat and distill off in vacuo the mixing solvent, i.e., NMP to obtain a polyaniline/carbon composite and use the composite, as an electrode for a capacitor. However, a polyaniline/NMP solution includes both a state where the polyaniline particles are dissolved in the NMP and a state where the polyaniline agglomerates are dispersed in the NMP. The above-mentioned polyaniline/carbon composite includes polyaniline agglomerates as well. The polyaniline agglomerates do not allow high speed, quantitative electrochemical reactions, and therefore, in the electrode using the above-mentioned composite, the composite polyaniline could not sufficient contribute to the improvement in the electrostatic capacity and it was difficult to improve the discharge capacity corresponding to the formation amount of the polyaniline composite.